Fly-by-wire control systems for aircraft have become increasingly prevalent as the accuracy and reliability of the processes of control have been extended by modern technology. Conventional-type backup controls add too much weight to the aircraft Therefore, a fly-by-wire backup, compatible with the main fly-by wire system becomes a necessity.
Past flight control devices, whether usable as a backup to a fly-by-wire system or not, include Soviet Patent No. 648,940 to A. I. Nikolaichenko entitled "Nonlinear Automatic Control System," which discloses a control scheme including an amplifier, differentiator, relay switch and comparator to produce a variable control when the actual error is greater than the set error and a linear control when the actual error is less than the set error.
U.S. Pat. No. 4,642,774 to Centala, et al, entitled "Automatic Flight Control with Pilot Fly Through" discloses a device which allows pilot deviation from a predetermined flight path and has a fading circuit to facilitate a default control scheme to the predetermined flight path once pilot intervention has ceased.
U.S. Pat. No 4,667,909 to Curci, entitled "Single Stick Control System for Helicopters" discloses a mechanical apparatus providing for manipulation of a single stick to effect directional flight, vertical flight and autorotation in the case of sudden power failure in flight. U.S. Pat. 4,696,445 to Wright, et al, entitled "Collective Control System for a Helicopter" discloses a circuit for transition of control between a force type stick and a displacement type stick to facilitate smooth transition therebetween.
U.S. Pat. No. 3,995,205 to Klees, entitled "Vehicle Maneuvering Control System" discloses a control system for use in controlling a vehicle using both manual and automatic control. U.S. Pat. No. 3,940,594 to Bleak, et al, entitled "Automatic Process Controller with Bumpless Operation" discloses a circuit for transfer between manual and automatic modes of control.
U.S. Pat. No. 2,974,908 to W. A. Platt, entitled "Closed Loop Ratio Changer and Automatic Trim Computer means for Controlling the Position of an Aircraft Control Surface" discloses a mechanical and electrical method for changing the response ratio to a surface to be controlled. U.S. Pat. No. 2,940,332 to W. D. Teague, Jr., entitled "Ratio Changer Device" discloses a mechanical method for changing the response ratio to a surface to be controlled.
U.S. Pat. No. 3,696,282 to Hirokawa, et al, entitled "Marine Autopilot System including Mode Engagement Features" discloses a comparator which generates a signal based on the differences between the set course signal and the heading signal. U.S. Pat. No. 2,859,926 to R. Westbury, entitled "Aircraft" discloses a standby mechanical linkage as a backup in the event of electrical failure.
U.S. Pat. No. 3,485,463, to R. J. Miller, entitled "Control Stick Transducer" discloses a device to provide a pair of electrical outputs, one proportional to stick displacement and the other proportional to stick position. U.S. Pat. No. 3,369,161, to L. A. Kaufman, entitled "Hard Over Attenuation Device for Servo System" discloses a device for mechanically limiting the range of movement for a servomotor.
U.S. Pat. No. 3,199,013 to Brahm, et al, entitled "Rate Responsive Compensation Change Circuit" discloses a stabilization system for a helicopter in both hover and forward flight in the yaw, pitch and roll axes. U.S. Pat. No. 3,528,633 to Knemeyer, entitled "System for Controlling and Stabilizing an Aircraft in Yaw" discloses a pilot assisted yaw control system which applies a control signal, upon pilot activation, which is proportional to the difference between the force applied and the yaw rate to thereby produce a yaw rate proportional to the applied force.
Most of the above references relate to transition "smoothing" during the changeover from one mode of control to another. Others relate to "controlling through" a given control echelon temporarily. None relate to a total failsafe backup control for fly-by-wire control failure. Problems of controllability exist in such back-up modes because there is poor position resolution in a compliant sidestick controller, and there is no method of maintaining trimmed flight other than trying to hold the stick displacement against the spring return force.